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'"68                                      AZYGOS VEIN

cavities of the mammal's heart, which fourfold division it shares
with birds alone, do not exactly correspond compartment for
compartment with those of the bird's heart, at least in so far as
concerns the ventricles. For the reptilian heart Is provided with
only one ventricle, and therefore the division of that cavity
must have been independently accomplished in mammals and
in birds.
There are two features in the venous system which distinguish
all tlie Mammalia (with the exception of JSchidna in one of
these points) from vertebrates standing lower In the series. The
hepatic portal system is limited to a vein which conveys to the
liver blood derived from the alimentary tract; in no' mammal
except in Echidna, is there any representative of the anterior
abdominal vein of lower vertebrates. In that animal there la
such a vein, which apparently arises from a capillary network
upon the bladder and passes up, supported by a membrane, along
the ventral wall of the abdomen to the liver, thus emptying
blood into that organ exactly as does the anterior abdominal
vein of the frog. In no mammal is there any trace of a renal
portal system. The kidneys derive their blood from the renal
arteries only.
Many mammals have two superior venae cavae ; this is the
case, for instance, in the Elephant and the Hodents and other
types lying comparatively far down in the series. In most if
not In all mammals there are considerable remains of one of the
posterior cardinals, in the form of the azygos vein, which opens
into the vena cava superior or pre-caval vein, i.e. the superior
cardinal just before the latter debouches Into the heart. This
one posterior cardinal Is usually on the right side; but it may
be on the left side, for instance in Tricfiosurus vulpecula. In
JETalmaturus ~bennettii there are two azygos veins, one left and one
right, of which the left is rather the larger.1
Urinary Organs.—The kidneys in the Mammalia have a
compact form, which contrasts with the somewhat diffuse and
vaguely-outlined kidneys of the Sauropslda. In mammals the
organ is as a rule of that peculiar shape which is called ** kidney-
shaped"; a depression termed the hHum, which receives the
ducts of the glands, indenting the border of an otherwise oval-
shaped gland. In some few mammals the kidney is broken up
1 Beddard, Plroc, Zwl. 8w. 180$, p. 1$0.